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Related to join forces: Canadian armed forces
join forces (with one)
To work together with one. If you two join forces, you might just win the election after all. I'm sorry, but you're going to have to join forces with Michael if you want to get this done.
join forces (with someone)
Fig. to combine one's efforts with someone else's efforts. The older boys joined forces with the younger ones to sing the school song. Let's join forces with the other faction and run our own slate of candidates.
Act together, combine efforts. For example, The public relations people joined forces to get better coverage for their candidates. This expression originally referred to combining military forces. [Mid-1500s]